1. Field of the Invention
The present invention relates to a lance system, for inter-tube inspecting and lancing as well as barrel spraying of heat transfer tubes of a steam generator in a nuclear power plant, in which foreign substances, such as sludge deposits piled up around heat transfer tubes, are inspected or removed by spraying high-pressure water.
2. Description of the Related Art
As well known to those skilled in the art, a nuclear power plant rotates a turbine by means of the force of steam generated by heating water using heat generated by nuclear fission of uranium, and operates a power generator using the above rotary force, thereby producing electricity. Heat transfer tubes are densely arranged in a steam generator. Since water of a high temperature, which is contaminated with radioactivity, flows inside the heat transfer tubes, and water of a low temperature, which is not contaminated with radioactivity, flows outside the heat transfer tubes, the above waters are heat-exchanged and the contaminated water is converted into steam of a high-temperature and a high-pressure. The force of the steam rotates the turbine and the generator, thereby generating electricity.
Sludge is piled up in the steam generator, as operating time goes by, deteriorates heat efficiency of the heat transfer tubes in the steam generator, and damages the heat transfer tubes, thereby shortening the overall life span of the steam generator. The above sludge mainly contains oxidized steel and oxidized copper, and is cohered, in case that the sludge is not removed from the steam generator, and is then stuck between the heat transfer tubes, thereby causing heat stress. Further, the sludge, together with water, flows, and wears the heat transfer tubes.
In order to reduce the negative influence of the sludge on the life span of the steam generator, there are suggested equipment and procedures for discharging a designated amount of cooling water from a steam generator to the outside during the operation of the steam generator. However, using these equipment and procedures, it is impossible to effectively remove sludge deposits from the steam generator.
Accordingly, manufacturers of steam generators recommend users to periodically lance the inside of the steam generator within a preventive maintenance and inspection period every year.
Conventionally, there are suggested various lance systems, which rectilinearly move back and forth along a no tube lane (also called a “blow down lane (BLD)”) at a central line of a steam generator and spray high-pressure water at an angle of 90 degrees in a moving direction. In the earlier stage of the inventions, a nozzle head, provided with two arrays of nozzles arranged apart at an angle of 180 degrees or less, which is attached to an end of a circular rod, was introduced. Here, high-pressure water ejected from both directions could cover all the regions of heat transfer tubes by rotating the circular rod from the outside of the steam generator.
However, in the above conventional lance systems, there exists a high possibility that the nozzle head experiences excessive vibration, when the nozzle head moves to the inner part of the steam generator, caused by an imbalance of the repulsive forces of the high-pressure water ejected from both directions during lancing, thereby being capable of seriously damaging the surfaces of the heat transfer tubes.
In case that the two arrays of nozzles attached to the nozzle head are arranged apart at an angle less than 180 degrees, strong repulsive force is imposed on the end of the circular rod, thus causing permanent deformation of the circular rod.
In order to overcome the above problems, one of the conventional lance systems employed a rigid guide support rail with a groove, which is tightly fixed by two hand holes on the wall of the steam generator spaced apart by an angle of 180 degrees or by a hand hole and a central support rod stationed near the center of the steam generator. The above lance system can move back and forth automatically along the guide support rail by the operation of a motor drive unit. That is, the lance system carries out lancing by ejecting high-pressure water by moving along the guide support rail. Thereby, it is possible to improve the overall efficiency of the lancing procedure without damaging the heat transfer tubes.
Recently, there is raised a necessity to remove hard sludge deposits, piled up around the heat transfer tubes, which are reported to be hard to remove by the above-described general lance method of ejecting high-pressure water at an angle of 90 degrees along the BDL. Many attempts to solve the above problem have been undertaken by EPRI, which is technically supported by Foster-Miller. The most effective methods were to increase the ejection pressure and the flow rate of the high-pressure water and to reduce the distance between nozzles for ejecting high-pressure water and targeted sludge deposits. The former had several technical problems, and the latter employed flexible means which could be bent by an angle of 90 degrees along the BDL so that the lance system could approach the inner parts of the heat transfer tubes.
That is, using the latter, the lance system could directly eject the high-pressure water just over the targeted sludge deposits.
The motivation and purpose of the application is quite similar to the latter, but the approaches to the inter-tube lance are quite different from the latter.